1. Field of the Invention
This invention relates to differential expansion rod and tube thermostats, and particularly to one used as a single or multiple point temperature limit switch.
2. Description of the Prior Art
The present invention relates to a thermally-responsive switch assembly, and particularly to a double-point, thermally-responsive safety control switch assembly for a self-cleaning electric oven to serve as a redundant switch to remove power from the oven heating units in case of a failure of the main oven control thermostat, and further to interrupt a door interlock circuit to prevent the opening of the oven door when the oven ambient temperature is above a predetermined level, and further to effect the operation of an indicator lamp and a cooling fan to maintain safe operating temperatures of the outer surfaces of the oven cabinet.
This invention is related to the art of pyrolytic, self-cleaning ovens which utilize a high temperature method of cleaning the interior walls of the oven cooking cavity of food soils and grease spatter by raising the temperature of the walls into a heat cleaning temperature range above about 750.degree. F. At this high temperature, the food soils are transformed into gaseous degradation products which are then oxidized and finally discharged from the oven cavity. The details of this method and of a preferred embodiment of an oven design in which it may be carried out, are fully disclosed in U.S. Pat. No. 3,121,158 of Bohdan Hurko, which is assigned to the same assignee as the present invention.
Thermally-responsive or thermostatic switches are commonly used in domestic appliances, such as baking and broiling ovens. Perhaps one of the most common types used is the hydraulic thermostat which is essentially a hydraulic-filled bellows or diaphragm assembly that converts volumetric expansion or contraction thereof into rectilinear motion acting upon a switch mechanism. Others are of the differential thermal expansion type such as a rod and tube thermostat as is shown in my earlier U.S. Pat. No. 3,821,683, which is also assigned to the present assignee. The thermostat of my earlier patent serves to translate a rise in oven temperature that is sensed by the rod and tube assembly into rectilinear motion which is translated through a ball and socket joint to a motion amplifier lever that in turn effects the opening and closing of a pair of switch members. These switch members are shown as slow make and break switches.
The thermal switch of my earlier patent operated in a control circuit for a self-cleaning oven, the same as used with the present invention; firstly, to insure that during the baking or broiling operations the oven air temperature does not rise above a safe temperature; for example, in the 600.degree. F - 650.degree. F. range in the event of a failure of one of the temperature control components; secondly, of insuring that the user may not unlatch the oven door during a self-cleaning oven cycle unless the oven ambient temperature returns below approximately 600.degree. F; thirdly, of insuring that the outside wall temperatures of the oven cabinet do not reach certain unsafe temperatures, and to effect this, at approximately 600.degree. F. oven ambient temperature, a cooling fan is energized to force room air to be circulated between the inner and outer walls of the oven structure.
The principal object of the present invention is to provide a thermally-responsive switch assembly using a rod and tube differential expansion thermostat which has ambient temperature compensation and also compensation for the thermal lag of this thermostat on the early heating cycles to prevent overshoot of the calibration temperature.
A further object of the present invention is to provide a rod and tube thermostat of the class described where the inner rod has an extension strap which is essentially outside the area with the temperature that is to be controlled, and this strap has thermal expansion characteristics similar to that of the tube.
A further object of the present invention is to provide the thermostat assembly of the class described wherein the extension strap couples the inner rod to an amplifier lever which serves as a switch actuator means.
A further object of the present invention is to provide a thermally-responsive switch assembly of the class described wherein the amplifier lever is provided with a hinging means that maintains a given spatial relationship between the base of the thermostat and the amplifier lever.
A further object of the present invention is to provide a thermally-responsive switch assembly of the class described combining a snap-action switch mechanism with the ambient temperature compensated rod and tube thermostat.